Passive surveillance is the most common type of surveillance in humanitarian emergencies. Most surveillance for communicable diseases is passive. The surveillance coordinator may provide training to health workers in how to complete the surveillance forms, and may even send someone to periodically collect forms from health facilities. But little attention is given to individual health workers who report the information.

The data requested of each health worker is minimal. Nonetheless, passive surveillance is often incomplete because there are few incentives for health workers to report.

This figure is an example of data gathered by passive surveillance from the hospitals run by one organization:

It is clear that the number of cases declined between July 2000 and February 2001.

Active surveillance

An active surveillance system provides stimulus to health care workers in the form of individual feedback or other incentives. Often reporting frequency by individual health workers is monitored; health workers who consistently fail to report or complete the forms incorrectly are provided specific feedback to improve their performance. There may also be incentives provided for complete reporting.

Active surveillance requires substantially more time and resources and is therefore less commonly used in emergencies. But it is often more complete than passive surveillance. It is often used if an outbreak has begun or is suspected to keep close track of the number of cases. Community health workers may be asked to do active case finding in the community in order to detect those patients who may not come to health facilities for treatment.

Sentinel surveillance

Instead of attempting to gather surveillance data from all health care workers, a sentinel surveillance system selects, either randomly or intentionally, a small group of health workers from whom to gather data. These health workers then receive greater attention from health authorities than would be possible with universal surveillance.

Sentinel surveillance also requires more time and resources, but can often produce more detailed data on cases of illness because the health care workers have agreed to participate and may receive incentives. It may be the best type of surveillance if more intensive investigation of each case is necessary to collect the necessary data. For example, sentinel influenza surveillance in the United States collects nasopharyngeal swabs from each patient at selected sites to identify the type of influenza virus. Collection of such data from all health workers would not be possible.

This figure is an example of data gathered by sentinel sites in Afghanistan:

From this figure, we can see that measles cases were distributed throughout Afghanistan in 2001.